Our Thermador wall oven will only heat to 315F. Fan works properly and so does the broiler. If we heat oven to 400 using the broiler set at 400 and then switch back to bake, the temp slowly drops to around 315. The bake element does get hot, but never red hot like it used to after the oven went through the preheat cycle. The display also reads "Preheat" the entire time it is in bake mode. We replaced the bake element in 2005 when it burned out. We also replaced the display with a replacement kit because you could not read any of the readings. Is the heating problem with the element, relay, circuit board, or some other component?